箭毒蛙生物碱Batrachotoxin的不对称全合成新策略初探——官能化的CD环的不对称合成

Exploratory Studies on a New Strategy for the Asymmetric Total Synthesis of the Arrow Poison-Frog Alkaloid Batrachotoxin:Asymmetric Synthesis of a Functionalized CD Ring

在已报道的800多种箭毒蛙素生物碱中, batrachotoxin因其极强的心脏和神经毒活性和复杂的分子结构引起了多学科的广泛关注.设计了一条与已报道的三条batrachotoxinin A的全合成路线不同的A环与CDE环通过Diels-Alder反应构建B环和整个分子中五环骨架的新策略.由于A环片段为已知化合物,CDE骨架的构筑成为本策略的关键.通过对这一策略即官能化的CD环的构筑的初步探索,首先合成了2-烯丙基-1,3-环戊二酮,然后与1-己烯-3-酮发生Michael加成反应得三羰基化合物.接着着重探索了基于该底物的Hajos-Wiechert型反应,以75%的产率和81%的ee值合成了烯丙基取代的CD环.后者经七步进行官能化,转化为CDE骨架的前体.

Among the more than 800 alkaloids isolated from skins of arrow poison-frog, batrachotoxin had attracted the most widespread attention of scientists from several fields, due to its potent cardio and nerve toxicities and challenging structure. A new strategy, which is different from all the previous total syntheses(one racemic and two enantioselective), is disclosed. This strategy features the formation of the CDE skeleton and then the ABCDE core of batrachotoxin by a key Diels-Alder reaction between A ring and CDE framework. Because A ring segment is a known compound, the first key of synthesis resides in the construction of the CDE framework. Thus, in addition to the new synthetic strategy, the synthesis of a functionalized CD ring system is developed. The synthesis started from the preparation of 2-allylcyclopentane-1,3-dione and its Michael addition with hex-1-en-3-one. The Hajos-Wiechert-type reaction of the adduct was investigated under optimized conditions using L-phenylalanine as an organocatalyst and D-camphorsulfonic acid(D-CSA) as an additive, the desired Robinson annulation proceeded smoothly to give the desired cyclization product in 75% yield and 81% ee. The latter was converted into a functionalized CD skeleton that bears all elements for elaborating to CDE framework in seven steps.